1. Field of the Invention
The present invention relates to an apparatus and a method for in situ treatment of a medium, especially where the medium is sludge.
2. Description of the Related Art
In the past, industries have discharged aqueous, dry or semi-solid waste materials, chemicals, heavy metals, or mixtures of toxic or radioactive materials into impoundment ponds as a sludge-type material. Typically, these materials consist of fine particulate matter that does not have good self-adhesion properties. Such waste material may leach into the soil and contaminate the ground water and may become dried out and solid, which may present an airborne health hazard.
A preferred method for lessening the danger inherent in impounded materials has been to stabilize the impounded waste material in place, i.e., in situ. In such an in situ method, the material is stabilized by transforming it into a solid, or soil-like, substantially insoluble mass from which toxic materials will not leach out, nor will any substantial surface particles of the mass become airborne when the mass is excavated or subjected to the elements of weather. Such a method and a device used to practice such method is described in U.S. Pat. No. 4,776,409 to Manchak and U.S. Pat. No. 4,652,180 to Jenkins.
One of the most critical aspects of stabilization is thorough, complete and uniform homogenization of the medium and the stabilization reagent. Reagents injected into waste material must be uniformly blended with the waste and must uniformly react with it; to do so, a homogeneous mixture must be produced. Otherwise, unstabilized portions of waste material may remain that constitute a health hazard, and excessive amounts of reagent must be used to stabilize the most highly-concentrated regions of the toxic waste. This results in substantial increases in cost and treatment time. While both Manchak and Jenkins stabilize waste material, only Manchak additionally homogenizes the material. However, Manchak achieves only limited homogenization. This is because, in Manchak's device, only one blade is disposed at the end of shafts, leaving the medium along the length of the shafts substantially untreated after the shafts are inserted vertically into the medium.
Another approach to in situ treatment of waste material is employed in a Deep Soil Mixing (DSM) system, commercially available from Geo-Con, Inc. of Pittsburgh, Pa. In this system, as the shafts are advanced vertically below the surface of soil in an impoundment pond, a cement-based grout is injected through the hollow stems of the shafts and is discharged at the lower end of the shafts (i.e., the end of the shafts inserted into the medium). The grout and the soil are thus mixed and stabilized, although not necessarily homogenized. Such a system is shown generally at 2 in FIG. 1. A disadvantage associated with this system is that to treat all of the volumes, the shafts must be raised above the surface of the soil, horizontally advanced and then lowered again vertically below the surface of the soil in an adjacent volume. In this process, areas must be overlapped. This overlap is shown at cross-hatched areas 4 in FIG. 1, and the pattern of the horizontal advancement of the cylindrical shafts of such a prior art system is shown in outline at 6. Without this overlap, a portion of the volume is left untreated because of the edge shape of the cylindrical volumes contacted by the blades mounted on the shafts. However, this overlap results in additional time and cost in treating the soil, which are critical factors in determining whether an impoundment pond can be rapidly, economically and completely stabilized.